User:Mater39100/sandbox/Single Atom Catalysts (SACs)

Overview
Single atom catalysts (SACs) are catalysts with atoms of catalytically active metals dispersed on a solid substrate. By virtue of being single atoms, the SACs do not suffer from dispersion related loss in selectivity. They offer the possibility of higher activity than normal nanomaterial-based catalysts due to their low coordination environments-having very few atoms or ions surrounding the central atom of the complex. They also provide the highest atom economy, since every atom is catalytically active. Atom economy, developed by Barry Trost, provides a measure for the number of atoms from a reactant that get incorporated into the final product. Most studies of SACs have employed noble metals because they are among the highest efficiency and selectivity due to low orbital energies and low redox potentials.

Introduction
Catalysis can be divided into two categories: homogenous catalysis, in which the catalyst is dispersed throughout the reaction system, and heterogeneous catalysis, in which the catalyst is confined to surfaces or interfaces. Homogeneous catalysis is effective at increasing the overall rate of reaction in a system, but this is offset by factors, such as the cost of separating the catalyst from the reaction mixture after the reaction. Generally, heterogeneous catalysis has less pure catalytic activity, but is much more applicable to industrial processes, and thus is more widely used in the petrochemical industry or in pharmaceutical synthesis.

As nanoscience was developed, nanoscale heterogeneous catalytic mechanisms was explored as an avenue to alleviate the cost burden of catalytic metals. This idea, taken to its most extreme, is single-atom catalysis. SACs have attracted much scientific interest, because they are distinctly different from traditional supported metal catalysts in that in SACs, bare metal atoms are atomically dispersed on the support, replacing the typical metal clusters or complexes. SACs, on the other hand, successfully circumvent this issue since the metal atoms are homogenously dispersed on a high surface area support and hence the active sites are all chemically equivalent. In general, SACs are widely regarded as unique from bulk materials for these characteristics: (1) low-coordination environment of metal centers, (2) quantum size effects, where electron confinement leads to discrete energy level distribution and a distinctive HOMO-LUMO gap, (3) metal-support interactions that allows charge transfer between metal species and supports, (4) foreign atom effect that produce asymmetrical spin and charge density. These characteristics are known to distinguish SACs from nanomaterial-based catalysts and give them unique capabilities for catalysis.

History
Catalysis has been fundamentally developed with the introduction of nanotechnology. The first use of the specific term “single atom catalyst” (SAC) was by Qiao and Zhang in 2011. However, SACs were being investigated and created for over a decade before this paper was published, although they weren’t specifically known by that name.

Over the past decades, much research effort has been directed toward optimizing industrial processes in terms of energy efficiency and environmental greenness. Catalysts provide a pivotal area of study for this purpose, since they are utilized in more than 90% of all industrial processes. The ultimate objective is to achieve catalysts that have high turn-over numbers for multiple cycles of use and can be readily separated from the reaction mixture1. Traditionally, homogeneous catalysts have more favorable activity and selectivity than heterogeneous catalysts; however, the process of recovery and recycling of homogeneous catalysts is challenging and presents a major limiting factor in their widespread utility. Thus, the application of heterogeneous catalysts dominate most industrial process; however, usefulness of heterogeneous catalysts has been hampered by from lower atom efficiency and product selectivity. It was long proposed that an ideal solution for all catalytic needs involved heterogenization of homogeneous catalysts, in which homogenous catalysts are anchored or dispersed on an insoluble substrate.

Atomic Cluster Catalysts
In 1988, John Thomas from Davy Faraday Research Laboratory first defined a new class of catalysts called ‘uniform heterogeneous catalysts’ which would represent the best of both heterogeneous and homogeneous catalysts. Following a reported work on single palladium atoms supported on thin magnesium oxide films in 2000, he renamed this class of catalysts single-site heterogeneous catalysts (SSHCs). SSHCs, according to Thomas, is categorized into four subclasses: (1) individual isolated atoms, ions or metallic clusters anchored to high-area supports, (2) asymmetric organometallic species immobilized on mesoporous solids, (3) “ship-in-bottle” structures in which isolated catalytic molecular entities are entrapped within zeolitic cages and (4) open, microporous solids or molecular sieves in which the isolated active sites are uniformly distributed spatially throughout the bulk and are located at or adjacent to ions that have replaced framework ions of the parent structure.

In 1991, zirconium hydride was placed onto a silica support, and was found to catalyze the hydrogenolysis of hydrocarbons like isobutane and propane, at temperatures lower than previously thought possible. This silica complex, although not fully characterized, could have been an early example of catalysis on the atomic scale. The same group also published a related paper in 1995, in which tantalum monohydride was created on a silica support and was experimentally measured with various types of spectroscopy. Although it is difficult to draw firm conclusions from these papers, it is possible that something similar to single-atom catalysis was occurring in these processes. In 2000, more solid connections to modern SACs can be found in a paper published by Abbet et. al. which is subtitled “One Atom Is Enough!”. In this paper, the authors specifically investigated how the size of small palladium clusters (between 1 and 30 atoms) affected the catalytic activity for the production on benzene. They found that benzene’s production was encouraged at 300 K for very small clusters of Pd, between 1 and 3 atoms. Surprisingly, one single Pd atom was observed to catalyze this production, and this experimental result was supported by density functional theory (DFT) calculations.

In 2005, Bohme amd Schwarz from York University proposed the feasibility of the first subclass of SSHCs defined by Thomas, single-site catalysts, via gas-phase experiments. With the advent of sophisticated characterization techniques and instruments, it became possible to detect and image at atomic levels. In 2005, Judai and Abbet expanded on this work. They generalized this catalytic activity of small clusters from one specific system to a more general description of catalysis. Specifically, by measuring turn-over frequencies, they investigated how the catalytic activity of small Pd clusters for NO reduction changed over the course of hundreds of reaction cycles. They found that these small clusters remained catalytically active over the course of a few hundred individual reaction cycles, and they are much more active at low temperatures than most bulk catalysts. This work specifically pointed out the possibility of changing the size of these small clusters of metal catalysts to tune both the catalytic activity and the chemical selectivity for various mechanisms.

Also in 2005, Böhme and Shwarz published a review on small clusters catalyzing various reactions in the gas phase, and Thomas et. al. published a review on single-site heterogeneous catalysis (SSHC), where small active sites are scattered across a support on an interface. These reviews emphasized the low-temperature catalytic activity and the chemical selectivity of these single sites or small clusters, but the idea of the catalysts themselves being single atoms wasn’t conclusively proven. After these reviews, the pace of scientific work on small clusters catalytically active metals accelerated, as these potential advantages of nanoscale catalysts were acknowledged.

In 2008, two similar papers were released on small gold clusters acting as catalysts. Turner et al. published a study in Nature that proved that small clusters (~55 particles) of gold atoms placed on a chemically unreactive support can selectively catalyze the oxidation of styrene. Furthermore, this chemical reactivity and selectivity was directly related to the size of the gold cluster itself, due to the electronic structure that the size imparted on the particle. The specifics of this electronic structure were not outlined, but the inherent importance of the size being small was explicitly pointed out. Around the same time, Herzing et. al. published an article in Science that reported the catalytic activity of even smaller clusters (~10 particles) of gold atoms in the oxidation of carbon monoxide. At this point, these catalytic clusters were found in more general systems, and the nanoscale size was found to be inherently important.

In 2009, Vajda et al published a paper on the catalytic activity and high selectivity of small platinum clusters (~8 particles) in dehydrogenation of propane. These small clusters were proven to be up to 400 times more catalytically active than the corresponding bulk heterogeneous catalyst, with extremely high chemical selectivity. These advantages were investigated through electronic structure calculations, and were determined to be due to the relatively high proportion of surface atoms that were not coordinated to other Pt particles. This level of incoordination increases as the size of the clusters decrease, as the fractional percent of particles on the surface of the cluster increases. In a similar vein, Lei and Vajda published an article in Science in 2010 that concluded that small Ag3 clusters on alumina supports highly catalyze the epoxidation of propylene. They suggest that small Ag clusters could be a new class of catalysts for epoxidation reactions.

Examples of Single-Atom Catalysts
Catalysis of CO oxidation using Pt/FeOx

CO oxidation is a very important process. All automobiles have attached catalytic converters that oxidize toxic gases like CO, nitrogen oxides (NOx) and unburnt hydrocarbons (HC) to harmless molecules. Modern catalytic converters employ a mixture of Pt, Pd, and Rh. Due to the increase in automobile use, the cost of production is increasing, and there is a need for more economically viable and efficient catalysts for CO and NOx reduction. Another important reduction of CO is Preferential Oxidation (PROX). In this reaction, the CO byproduct formed during steam reforming of hydrocarbons to hydrogen, is preferentially reduced to carbon dioxide. This reduction is important, since the resulting gas is used as the hydrogen feed for fuel cells, and CO would deactivate the catalyst employed in the cell.

In 2011, Qiao and Zhang published a benchmark paper where a single Pt atom was proven to be responsible for the oxidation of carbon monoxide. This activation was shown to be due to the partially empty 5d orbitals of the Pt atoms that were individually isolated onto an iron oxide nanocrystal. This had to be due to SACs themselves, as there were no small clusters present in the system to act as the catalyst. This paper, for the first time, introduced the term single-atom catalysis, which has since been formalized into a specific sub-field of catalysis. There has been much work on SACs since this paper, specifically on their various applications and on how to efficiently synthesize them.

Hydrogenation of 1,3-butadiene using Pt single atoms on Cu Nanoparticles

Atoms can be attached to the surface of nanoparticles (NPs) to form single atom catalysts. These systems are called single-atom alloys (SAA). One of those single-atom alloys is used to catalyze the hydrogenation of butadiene. This reaction is a good model system for selective catalysis of dienes. It is also a convenient model system for catalytic hydrogenation of olefins, which has a big role in the food industry. The reaction was done in relatively mild conditions (380K and atmospheric pressure). The authors report binding of Pt atoms onto the plane, xyz orientation of the planes in the lattice based on Miller indices, of Cu NPs. The authors report selective hydrogenation of butadiene by the catalyst, without hydrogenation of unconjugated alkenes (<1% propene in the mixture was reduced). This is in sharp contrast to bulk Pt catalysts, which are known to over-hydrogenate dienes and alkynes. The study also shows that there is minimal formation of CO2 and oligomers, which indicates that there is no decomposition of butadiene and the reaction is highly selective. The authors also demonstrate that Pt/Cu SAA bind less strongly to CO than bulk Pt. This demonstrates that Pt/Cu SAA are more robust and less susceptible to poisoning and are more feasible for fuel cell applications.

Reduction of alkynes and nitroarenes using Pd SAC on mesoporous graphitic carbon nitride

Selective reductions of individual functional groups like triple bonds and nitro groups are important tools in organic chemistry. Traditional methods for selective reduction of alkynes to alkenes involve using Pd or Pt nanoparticles embedded on a substrate, poisoned by lead (Lindlar’s catalyst). The use of lead makes the reaction harmful to the environment. There is a need for eco-friendly alternatives for these reactions. Single atom catalysis of Pt atoms anchored onto mesoporous graphitic carbon nitride is reported to catalyze 1-hexyne reduction at 303K and 1 bar with activity three orders of magnitude higher than reference nanoparticle systems based on Au, Ag and CeO2. At 5 bar and 343K, the SAC is 4 times more active than the reference Pd-based Lindlar’s catalyst, while displaying equally high selectivity of 90% with the reference catalyst. The SAC also showed a high selectivity towards cis olefin, with a product cis/trans ratio of 20, and selective reduction of the triple bond in molecules having alcohol groups. Side reactions like adjacent hydride abstraction were not observed, indicating that the selectivity is robust.

Design of Single Atom Catalysts
Isolated metal atoms have much higher surface energy than their nanoparticle or cluster counterparts. As a consequence, they tend to aggregate during the construction process or post-synthetic treatment process, presenting a challenge in SAC fabrication. During the past decade, several approaches have been reported to stabilize SACs, which can be broadly categorized into (1) enhancing metal-support interaction, (2) optimizing anchor sites on the support, and (3) point defects on supports.

(1) Mutual metal-support interaction

Mutual interaction between the isolated atoms and supports influences the catalytic selectivity, activity and stability of the metal atoms. The coordination with the support also changes the electronic properties of metal catalysts and the strength of coordination determines the efficiency of catalytic processes. Therefore, careful selection of support material is important for the overall performance of SACs.

(2) Available anchor sites

The support stabilizes the isolated metal atoms via specific anchoring centers that can even trap mobile ions at high temperatures. The surface area of the support also determines the loading capacity of the metal atoms. Interaction of metal atoms with their anchor sites can also allow performance at high temperatures without leading to desorption of catalytic centers. With stable binding sites, SACs can retain its atomically dispersed structure and resistant to sintering at high temperature reactions.

(3) Coordination geometric effects

To ensure the homogeneity of the catalytic centers, every metal atom should be in the same chemical environment, i.e., it should have the same coordination geometry and number, to retain the advantages of both homogenous and heterogeneous catalysts. The coordination geometry of the atoms plays a vital role in determining the final catalytic performance such as selectivity and atom efficiency. The geometrical pattern of the support can therefore be constructed and tuned according to the desired number and type of coordination sites bonded to the active metal centers.

Preparation
A few synthetic routes have been proposed to construct SACs. The more commonly utilized approaches include: (1) high-vacuum physical deposition methods, (2) wet-chemistry methods, (3) modular synthetic methods, (4) atomic layer deposition method.

(1) High-vacuum physical deposition methods

In this approach, also known as the soft-landing technique, low energy mass-selected ion beams are directed at metal surfaces to achieve precise modifications at the surfaces. It allows precise control of the metal species and regulation of the surface structure of the support. This method is used for fundamental studies but is not optimal for large scale syntheses of SACs due to the high cost and low yield associated with this fabrication process.

(2) Wet-chemistry methods

In this technique, metal atom precursors are introduced to the support via ion-exchange/impregnation, deposition-precipitation, or coprecipitation process. Afterwards, the metal loaded support is dried and calcined, followed by reduction or activation procedures as necessary. While this approach is scalable, care must be taken to avoid degradation and/or aggregation of metal species on the support during the post-treatment processes. A strong metal-support interaction is required, and typically only low loadings of metal are obtained. This approach is more common for commercial production of supported metal catalysts.

(3) Modular synthetic methods

This approach utilizes the metal-ligand interaction to anchor metal atoms at predefined locations/configurations. This requires the support to have clearly defined coordination sites and space to support metal atoms in a monodisperse manner. In this case, the choice of support material is crucial for the loading efficiency and activity of the atoms. Metal organic frameworks (MOFs) are one such class of support material for SACs. MOFs are composed of metal ions or clusters linked together by organic ligands to give an extended repeated cage-like structures in 3D. MOFs provide ideal binding sites for metal atom catalysts due to their periodic structure, porosity, and structural tunability. SACs with MOFs support have shown superior activity and have added advantages of being able to be recycled multiple times. However, typical MOFs are not stable above 350°C in air, limiting the application of such SACs from many reactions that require high temperatures. In a related class of materials, metal-organic complexes (MOCs), similar to MOFs, are also built from metal nodes and organic ligands. In this case MOCs directly undergo pyrolysis that modifies the metal nodes into metal catalysts. The metal nodes can also be exchanged with other metals to give diverse catalysts. Challenges in this approach include the difficulty of producing monodisperse particles and a complete conversion of metal nodes to the desired modifications.

(4) Atomic layer deposition method (ALD)

Atomic layer deposition relies on sequential self-terminating reactions between a solid surface and gas phase precursor molecules. In ALD, the support material is exposed to pulsing vapors of alternate precursor species. With each pulse, the precursor molecules react with the surface in a self-limiting way in that the reaction self-terminates once all the reactive sites on the surface are depleted. This allows the reactions to afford homogenous depositions over large-surface-area porous supports. With this approach, SACs can be constructed with high precision by controlling the structure and composition of related support. This technique has been utilized to synthesize isolated metal atoms on graphene or other types of supports.

Characterization of Single Atom Catalyst
Continued progress in design, synthesis, and understanding of single atom catalysts (SAC) requires thorough characterization of their structures. Characterization of single-atom catalysts involves probing the single atoms, their clustering, and their catalytic activity. Among the most frequently used methods to characterize SAC are electron microscopy and spectroscopic techniques.

1 Electron Microscopy

1.1 Scanning transmission electron microscopy (STEM)

High-angle annular dark field (HAADF)imaging can distinguish atoms based on atomic number and is used to identify individual metal atoms in STEM images. In the HAADF-STEM analysis, the catalytic metal atoms, which are typically noble metals, have a higher atomic number and appear bright and highly contrasted from their supports, which are often composed of lighter elements such as metal oxides or carbon. Single atoms common catalytic metals including Pd, Au, Ag, and Fe have been imaged. A drawback of HAADF-STEM is its limited ability in distinguishing single metal atoms with close atomic numbers.

1.2 Scanning tunneling microscopy (STM) 

Another imaging method with angstrom-level resolution is Scanning Tunneling Microscopy (STM). While STM is limited to materials in which the underlying substrate is conductive, it has the benefit of having both lateral and depth resolution; that is, it is a surface imaging method. It has been used to study metal-on-metal single-atom catalysts.

2 Spectroscopic Techniques

2.1 X-ray absorption spectroscopies

X-ray absorption spectroscopies here refer to X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy. X-ray absorption spectroscopy sheds X-rays of a narrow energy resolution on the sample and detects the intensity of the transmitted signals. EXAFS focus on the absorption energy of about 30 to 1000 eV above the absorbing edge energy, while XANES detects energy lower than 60 eV above the absorbing edge energy.

For SACs, the EXAFS conducts with the X-ray energy corresponding to the K-edge of the SAC center element. Basically, the oscillation manners of the SAC with substrate are different from the pure elements because of the different coordination environment of the center atom.

In XANES spectra, the absorption edge data shows the state of charge of the SAC center element, thus could determine the formal oxidation state of the center element.

2.2 Fourier-transform infrared spectroscopy

Usually CO is used as a probe molecule to detect the metals acquiring a relatively strong CO absorption. In situ FT-IR combined with CO absorption is applied to characterize the surface configuration of SAC. Different wavelength of absorption peak corresponds to the different coordination geometry of center atom.

2.3 Nuclear Magnetic Resonance The NMR spectroscopy is used to reveal the details of anchoring sites of SAC center atom within the substrate. The different chemical shift indicate a difference in the center atoms coordination environments and saturation level.

2.4 Characterization of Catalytic Activity 

Traditional chemical methods of monitoring reactions are critical for measuring catalytic activity and selectivity of single-atom catalysts and demonstrating their advantages over commercially used catalysts. These analytical methods have included gas chromatography, and continuous emissions monitoring systems (CEMS) analyzers. For example, in 2015, scientists developed a single-atom Pd1/graphene catalyst for 1,3- butadiene hydrogenation to 1-butene. The hydrogenations were monitored using gas chromatography, and the researchers theorized based on these results that the smaller Pd surface promoted butadiene mono-π-adsorption over di-π-adsorption, disfavoring hydrogenation of both double bonds.